Add visitors that throw by default

pkg/analyzer/lib/dart/ast/ast.dart

Index: pkg/analyzer/lib/dart/ast/ast.dart
diff --git a/pkg/analyzer/lib/dart/ast/ast.dart b/pkg/analyzer/lib/dart/ast/ast.dart
index 78b3c0668b5217a2130b381489b26bad25f2810d..c8a8cd8fd2051266722b1e10d775f95c30240478 100644
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@@ -556,7 +556,7 @@ abstract class AstNode implements SyntacticEntity {
 /**
  * An object that can be used to visit an AST structure.
  *
- * Clients may extend or implement this class.
+ * Clients may not extend, implement or mix-in this class.

[Paul Berry, 2017/01/09 17:36:51]

Consider adding a reference to ThrowingAstVisitor here, so that clients who
previously extended or implemented this class will have a hint as to what they
should do instead.

[Brian Wilkerson, 2017/01/09 18:08:07]

Done

  */
 abstract class AstVisitor<R> {
   R visitAdjacentStrings(AdjacentStrings node);